I don't think we should provide too many provider out of the box as it will slow our capability to provide new features. Instead we should rely on the community to bring that support. In addition, most providers have an smtp api, so our smtp provider can be used as a fallback.
That said, if we want to extend the feature coverage of our abstraction, this might make sense to have 2-3 commercial alternatives. We should select them based on Quality Of Server, Features and Market Share. This article provides a few insights.